A method which simply estimates the sleep quality during sleep has been suggested in, for example, Patent Literature 1 or the like. In the method of Patent Literature 1, a sleep state is estimated from a physiological index. In the method of Patent Literature 1, the rhythm of sleep of a subject is estimated from the appearance probability of a specific sleep stage per unit time. In the method of Patent Literature 1, the sleep quality, such as “sleep rhythm”, “head relaxation”, and “quick falling asleep”, is estimated using the rhythm of sleep of the subject. A sleep stage II and subsequent stages become SWS (deep sleep), and the sleep quality is estimated from the appearance probability of SWS. In the method of Patent Literature 1, for example, when an evaluation element is “head relaxation”, a period during which the appearance ratio of SWS increases from 0% after going to bed, the appearance ratio of SWS exceeds a predetermined threshold level S1, such as 40%, and becomes 0% again, and then, the appearance ratio of SWS increases becomes one cycle. In the method of Patent Literature 1, an evaluation point is calculated according to the total of the appearance ratio of SWS within one cycle for every cycle, and the sum of the evaluation points of the respective cycles becomes the evaluation of “head relaxation”.